Modern sputter coating devices are generally of the magnetron type, including a target cathode, an anode and a plasma having positively charged ions that bombard a surface of the target. In response to the bombardment, atoms are sputtered from the surface and propagate toward a substrate which is coated thereby. The plasma is formed by ionizing electrons moving between the cathode and anode through an inert gas. The ionizing electrons have a tendency to be confined in proximity to the target emitting surface by a magnetic field having lines of flux adjacent the target emitting surface. The confining magnetic field is at right angles to electric field lines extending between the anode and cathode. The right angle relationship between the electric and magnetic fields produces a magnetron effect of spiraling charged particles in the vicinity of the target.
In response to the target being bombarded by ions in the plasma, the target has a tendency to erode. Erosion of the target is non-uniform as a function of position on the target surface relative to an established plasma position. The non-uniform erosion of material from the target changes the path of material sputtered from the target to the substrate. Thus, the profile of sputtered material deposited on the substrate changes as a function of the amount of time that a particular target is in use.
It has been previously realized that the profile of sputtered material deposited on the substrate is a function of the profile of target erosion, which is in turn a function of the impedance of the plasma. It has also been previously realized that, to a certain extent, variations in the profile can be compensated by changing the plasma impedance. The plasma impedance is known to be a function of (1) the geometry of the sputter coating apparatus, (2) characteristics of the inert gas, e.g. the gas pressure, the ionization potential of the gas, contaminates of the gas and (3) the magnitude and geometry of the magnetic field. It has generally been assumed in the prior art that the sputter coating geometry is constant. In the prior art, the pressure of the inert gas has been controlled to compensate for expected changes in the profile of sputtered material attendant with erosion of the target. The magnitude and geometry of the magnetic field relative to the target base varied in an uncontrolled manner.
It has also been suggested in the commonly assigned U.S. Pat. No. 4,166,783 to Frederick T. Turner that compensation for aging and deterioration of the target can be controlled by a computer which responds to desired deposition rate information to control a plasma discharge. The computer regulates operation of a closed loop system wherein the power dissipated by an excitation source for the plasma is monitored from current drawn from the cathode and the cathode-anode voltage. The power dissipated in the sputtering discharge is corrected to maintain the desired deposition rate from the target cathode. The computer monitors an indication of the cumulative history of the target to update the correction periodically. The desired deposition rate is derived from information supplied to the computer independently. The computer determines a reference deposition rate level to which operation is stabilized. The computer determines from objective criteria based upon experience with a particular type target when the end of the useful life of a particular target cathode is reached.
While the prior art systems operate satisfactorily to a certain extent, there are refinements which can be made in controlling the magnetron sputter coating process, particularly for automatic control purposes. In the Turner patent, to correct for cathode erosion, the desired deposition rate, length of time that the cathode has been used, power applied to the plasma and the plasma pressure are assumed to be interrelated to control the power applied to the cathode but there is no on-line, closed loop control of the plasma impedance, although it is known that impedance changes with erosion because of changes in the position and geometry of the magnetic field relative to the eroded target surface. In numerous situations, it is desirable to provide online control for the plasma impedance because of variations associated with both the inert gas pressure and the magnetic field magnitude and configuration.
The magnetic field has a particular tendency to change during normal usage when the target is made of a magnetic material, i.e., a material having a high magnetic permeability. Magnetic targets are currently used for several purposes, one of which is to assist in the manufacture of magnetic disc memories having very high data density. To precisely control the thickness and uniformity of magnetic materials deposited on non-magnetic, generally aluminum substrates employed for magnetic disc memories, it is extremely important to control the manner in which the target material erodes, to control the deposition uniformity on the substrate throughout the life of a target.
The deposition profile of the sputtering material on the substrate can be maintained uniform as the target erodes only by control of the plasma impedance, unless complicated and expensive means are employed to manipulate the substrates during deposition. Control of the plasma impedance of a magnetic target is complicated because the magnetic material affects the amplitude and shape of the confining magnetic field. The magnetic target has a tendency to be a variable reluctance shunt for the magnetic field.
The reluctance of a magnetic circuit including the magnetic target has a tendency to change as a function of target temperature and erosion. When a relatively high magnetic permeability target is initially installed and is approximately at room temperature, it has a very low magnetic reluctance so that virtually all of the magnetic field applied thereto flows through the target and a fringing field in the target vicinity is relatively low. As the temperature of such a target increases, the magnetic permeability of the target has a tendency to decrease, due to the Curie point effect. The decrease in magnetic permeability due to the Curie point effect is a gradual phonomenon, whereby permeability decreases as a continuous function with increasing temperature, until a certain temperature is reached at which the target magnetic permeability drops virtually to zero. As the target erodes, the target crosssectional area decreases, with a resulting increase in magnetic flux density, i.e., B=.PHI./A increases because A decreases,
where: B equals magnetic flux density in the target,
.PHI. equals total magnetic flux applied to the target, and
A equals cross-sectional area of the target through which the magnetic flux flows.
The Curie point and erosion effects materially change the nature of the confining magnetic field in the vicinity of the target. As the temperature of the target increases, causing the magnetic permeability of the target to decrease, the target reluctance decreases; as the target reluctance increases there is a resulting increase in the fringing field in the vicinity of the target. Similarly, the decreased target area accompanying the target erosion causes the target reluctance to increase, with an increase in the fringing field in the target vicinity. As the fringing field in the target vicinity increases, the plasma in proximity to the target emitting surface is confined to a greater extent, i.e. to a smaller volume, because the magnetic field through the target decreases as the fringing magnetic field gradient increases. Thereby, the plasma impedance changes.
It is, therefore, an object of the present invention to provide a new and improved method of and apparatus for controlling a sputter coating magnetron depositing apparatus.
Another object of the invention is to provide a new and improved method of and apparatus for controlling a magnetron sputter coating depositing apparatus including a target that erodes as it is used during deposition.
Another object of the invention is to provide a new and improved method of and apparatus for controlling a magnetron sputter coating depositing apparatus wherein the tendency of a distribution pattern of atoms on a target substrate to change as the target erodes is compensated so as to be substantially overcome.
Another object of the present invention is to provide a new and improved apparatus for and method of controlling a magnetron sputter coating apparatus for depositing magnetic materials on a substrate.
Another object of the present invention is to provide a new and improved method of and apparatus for controlling a magnetron sputter coating depositing apparatus wherein the tendency of a magnetic target cathode to change the characteristics of a confining magnetic field as the target is eroded is compensated.
Another object of the present invention is to provide a new and improved apparatus for and method of compensating for the tendency of the reluctance patterns for magnetic target cathodes of magnetron sputter coating depositing apparatus to change as the target cathode is used.
Another object of the present invention is to provide a new and improved apparatus for and method of controlling a magnetron sputter coating depositing apparatus including a magnetic target cathode which has a tendency to have a variable reluctance path due to temperature variations.
Another object of the present invention is to provide a new and improved apparatus for and method of controlling a magnetron sputter coating depositing apparatus including a magnetic target cathode which has a tendency to have a variable reluctance path due to target erosion.